Trouble shooting my gurgling sink and tub on upper level bathroom?

My bathroom sink gurgles after I run water down it. It is a pedestal sink. previously I used to be able to snake up the vent pipe and it would solve the problem for a few months. This time, it is more difficult. I've run water down the stack from the roof (maybe it wasn't enough water-it was only for about 3-5 minutes). The snake method used to work, but it is such a tight area (the sink is in an area shaped lik a 'U' so it is a snug fit, so it is difficult to work behind the sink as it is a snug fit, and snaking it is always a challenge, because it is a lot of work getting the snake up the pipe. I think it may be because of the joints leading up to the roof, or possible clogs, it is hard to say, but a challenge to snake regardless; also the tub will also gurgle occassioanlly, and it seems that the gurgling is lasting longer then it used to. Any chance it could be a blockage leading down to the septic (which is pumped regularly-so it's not the septic) if so how could I get to that blockage, I'm still new to the house, and am not familiar with plumbing. Any ideas?? thanks